Magnetic sheets containing flaky soft magnetic powder have been used in electromagnetic wave absorbers and antennas for radio frequency identification (RFID). In recent years, such a magnetic sheet has also been used in a position detecting device, which is called “digitizer.” JP2011-22661A (Patent Document 1) discloses an electromagnetic induction-type digitizer including a pen-shaped position indicator and a panel-shaped position detector wherein a high-frequency signal transmitted from a coil embedded in the tip of the position indicator is read by a loop coil embedded in the position detector, to detect the indicated position.
A sheet serving as a magnetic path for the high-frequency signal is disposed behind the loop coil for enhancing the detection sensitivity. The magnetic path sheet is composed of, for example, a magnetic sheet prepared by orientation of flaky soft magnetic powder in a resin or a rubber, or a sheet prepared by bonding of soft magnetic amorphous alloy foils. In the case of the use of such a magnetic sheet, the entire detection panel can be composed of a single sheet. Thus, the magnetic sheet exhibits detection uniformity superior to that of the sheet of amorphous alloy foil, which may cause poor detection at the bonding portion.
A traditional magnetic sheet contains powder of an Fe—Si—Al, Fe—Si, Fe-Ni, Fe—Al, or Fe—Cr alloy flattened with, for example, an attritor (attrition mill) for the following reasons. As indicated by the “Ollendorff formula,” the preparation of a magnetic sheet having high magnetic permeability requires the use of soft magnetic powder having high magnetic permeability, the use of flaky powder having a high aspect ratio in a direction of magnetization for reducing the demagnetizing field, and the filling of the magnetic sheet with soft magnetic powder at high density. In particular, a high aspect ratio is considered an important factor, and thus powder is often flattened with an attritor under conditions for maximizing the aspect ratio.
Japanese Patent No. 4636113 (Patent Document 2) discloses a method of producing a flaky soft magnetic powder having a large major-axis length and a high aspect ratio, the method involving flattening of powder in the presence of a monohydric alcohol having two to four carbon atoms as an organic solvent.